Electrophotography has been broadening its application in the field of copying machines, laser beam printers, etc. because of the advantages of high speed and high image quality.
Photoreceptors conventionally widespread in electrophotography are those comprising inorganic photoconductive materials, such as selenium, selenium-tellurium alloys, selenium-arsenic alloys, and cadmium sulfide.
Organic photoreceptors comprising organic photoconductive materials have also been studied with attention being paid on their merits over inorganic photoreceptors, such as cheapness, productivity, and ease of disposal. In particular, organic photoreceptors of separate function type having a laminate structure composed of a charge generating layer which functions to generate charge on exposure to light and a charge transporting layer which functions to transport the generated charge are excellent in electrophotographic characteristics, such as sensitivity, charging properties, and stability of these properties on repeated use. Various proposals on this type of photoreceptors have been made to date, and some of them have been put to practical use.
While organic laminate type photoreceptors with sufficient performance in terms of the above-mentioned electrophotographic characteristics have been developed, there still remains an unsolved problem of durability against outer force in nature of the organic materials used. That is, being made of an organic material, a photosensitive layer easily undergoes wear or scratches on contact with a toner, a developer, a transfer medium (e.g., paper), a cleaning member, and the like and tends to cause image defects due to a toner filming phenomenon, adhesion of foreign substances, etc. Besides, low-resistant substances, such as corona discharge-induced ozone and nitrogen oxides, and paper dust from copying paper are liable to adhere and be deposited on the surface of the photoreceptor, which leads to image running under a high humidity condition. The working life of an organic photoreceptor has been considerably limited by these phenomena.
On the other hand, with the recent advancement in color image formation and high-speed recording with copying machines and color printers, the process involved has been getting more complicated, and the stress imposed on a photoreceptor has been increasing. From this viewpoint, the demand for improved durability of an electrophotographic photoreceptor has been increasing.
A number of measures have ever been proposed to improve durability of an electrophotographic photoreceptor. For example, polymethyl methacrylate resins, polyester resins, polycarbonate resins, etc. have been suggested as a binder resin for the surface layer of a photoreceptor (see JP-A-60-172044, JP-A-62-247374, JP-A-63-148263, and JP-A-2-254464; the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Use of these known binder resins somewhat achieved improvement on durability, but the state-of-the-art photoreceptors are still unsatisfactory. That is, the coating film comprising the known binder resins does not always have sufficient mechanical strength and, when repeatedly used in a copying machine for a long time, it reduces its thickness due to abrasion and thereby reduces its sensitivity. It follows that fog develops or the charging potential decreases to reduce the image density. Further, the scratch made on the surface of a photosensitive layer or a foreign substance adhered thereto due to toner filming, etc. causes image defects.
Where a photosensitive layer has a charge transporting layer comprising a charge transporting material and a binder resin, compatibility between the binder resin and the charge transporting material is of importance. If the compatibility is poor, it is known that the charge transporting material crystallizes to give serious influences on electric characteristics and image characteristics. Therefore, binder resins to be used in a charge transporting layer must have excellent compatibility to a charge transporting material used in combination.